Power amplifier circuits are commonly used in a variety of applications for a number of purposes, including to apply gain to a signal to generate an amplified output signal. For example, cellular telephone communications, high-speed data communications, and other applications typically include transmitters with power amplifiers. In some cases, it is desirable to ensure that signals are amplified by the power amplifier without distortion. For example, when the signal power exceeds a linear operating range of the power amplifier, distortion can result, which can be undesirable.
For example, many modern communications schemes generate time-domain signals that manifest a high Peak to Average Power Ratio (PAPR). For example, some modulation schemes can sum frequency components in a manner that results in a highly variable amplitude, resulting in a high PAPR in the transmitted signal. Such high-PAPR signals can be particularly prone to distortion when passed through a power amplifier, and many applications that include such high-PAPR signals can be particularly sensitive to the resulting distortion.
As such, it is typically desirable in such applications to mitigate distortion of high-PAPR signals in the power amplifier of the transmitter. Various conventional approaches are used to avoid such distortion. For example, some conventional approaches dynamically reduce the gain of the signal at the input to the power amplifier as the power of the signal increases, which can help ensure that the signal power does not exceed the linear operating range of the power amplifier. However, such approaches tend to appreciably reduce the efficiency of the power amplifier. Other conventional approaches use hard clipping to effectively keep the signal amplitude from exceeding a threshold limit. However, hard clipping can tend to produce hard edges in the signal, which can cause out-of-band emission in the transmission. Some such approaches can use filtering to smooth the hard edges produced by the hard clipping, but the filtering can have its own limitations. For example, some such filtering approaches are relatively complex, resulting in more expensive circuitry. Further, some such filtering approaches can still result is undesirable signal characteristics, such as by generating signal peak regrowth in excess of the threshold limit.